1. Field of the Invention
The present invention relates to CMOS-based color pixels and, more particularly, to a CMOS-based color pixel with reduced noise in the blue signal.
2. Description of the Related Art
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting a pixel of light energy into an electrical signal that represents the intensity of the light energy. In general, CCDs utilize a photogate to convert the light energy into an electrical charge, and a series of electrodes to transfer the charge collected at the photogate to an output sense node.
Although CCDs have many strengths, which include a high sensitivity and fill-factor, CCDs also suffer from a number of weaknesses. Most notable among these weaknesses, which include limited readout rates and dynamic range limitations, is the difficulty in integrating CCDs with CMOS-based microprocessors.
To overcome the limitations of CCD-based imaging circuits, more recent imaging circuits use active pixel sensor cells to convert a pixel of light energy into an electrical signal. With active pixel sensor cells, a conventional photodiode is combined with a number of active transistors which, in addition to forming an electrical signal, provide amplification, readout control, and reset control.
FIG. 1 shows a cross-sectional and schematic diagram that illustrates a portion of a conventional CMOS active pixel sensor cell array 10. As shown in FIG. 1, array 10 includes a plurality of active pixel sensor cells 12 which are formed in rows and columns, and a plurality of column sense amplifiers 14 which are connected to the cells 12 so that one amplifier 14 is connected to each cell 12 in a column of cells.
As further shown in FIG. 1, each cell 12, which is formed in a lightly-doped, e.g., 1.times.10.sup.14 to 1.times.10.sup.15 atoms/cm.sup.3, p-type substrate 16 (or an epitaxial (epi) layer), includes a heavily-doped, e.g., 5.times.10.sup.15 to 1.times.10.sup.19 atoms/cm.sup.3, n+ region 18 which is formed in substrate 16 (or the epi layer) to function as a photodiode, and a reset transistor 20 which has a source connected to n+ region 18 and a gate connected to receive one of a plurality of reset signals RST1-RSTn.
In addition, each cell 12 also includes a source-follower transistor 22 which has a gate connected to n+ region 18, and a row-select transistor 24 which has a drain connected to the source of source-follower transistor 22 and a gate connected to receive one of a plurality of row-select signals RWSL1-RWSLn.
In operation, array 10 first sequentially resets each row of cells in the array. During the reset step, the potential on n+ region 18 of each cell 12 in the first row of cells is raised to an initial transfer voltage by pulsing the gates of the reset transistors 20 in the first row with a positive voltage via the reset signal RST1. The initial transfer voltage placed on n+ region 18 of each cell 12, in turn, defines an initial intermediate voltage on the source of each of the source-follower transistors 22 in the first row.
Immediately after the gates of the reset transistors 20 have been pulsed, the gates of the row-select transistors 24 in the first row are pulsed with a positive voltage via the row-select signal RWSL1. The positive voltage on the gates of the row-select transistors 24 causes the initial intermediate voltages on the sources of the source-follower transistors 22 to appear on the sources of the row-select transistors 24 as initial integration voltages.
The initial integration voltage of each cell 12 in the first row is sensed and amplified by the column sense amplifier 14 that corresponds with each cell 12 in the row, and then stored by an imaging system (not shown). The same process is then repeated on the remaining rows in the array.
Following reset, light energy, in the form of photons, penetrates into n+ region 18, and substrate 16 (or the epi layer) of each cell 12, thereby creating a number of electron-hole pairs in each cell 12. The photogenerated holes which are formed in n+ region 18 of a cell 12 which diffuse over to the junction region are attracted to substrate 16 (or the epi layer) of the cell 12 under the influence of the junction electric field, while the photogenerated holes formed in substrate 16 (or the epi layer) of the cell 12 remain in substrate 16 (or the epi layer).
Similarly, the photogenerated electrons formed in substrate 16 (or the epi layer) of the cell 12 which diffuse over to the junction region are attracted to n+ region 18 of the cell 12 under the influence of the junction electric field, while the photogenerated electrons formed in n+ region 18 of the cell 12 remain in n+ region 18 where each additional electron reduces the potential on n+ region 18 of the cell 12.
Thus, at the end of an integration period, the potential on n+ region 18 of each cell 12 in the first row will have been reduced to a final transfer voltage where the amount of the reduction represents the intensity of the received light energy.
Following this, the final transfer voltage on n+ region 18 of each cell 12 in the first row is then read out via the source-follower transistors 22 of each cell 12 as a final integration voltage by again pulsing the gates of the row-select transistors 24 in the first row. As a result, the total charge collected by each cell 12 in the first row is determined by subtracting the final integration voltage of the cell from the initial integration voltage of the cell.
With black-and-white imaging, each cell 12 represents a pixel of light energy. However, with color imaging, three cells 12 are required to form a single color pixel. FIG. 2 shows a cross-sectional and schematic diagram that illustrates a conventional color pixel 50.
As shown in FIG. 2, color pixel 50, which uses the same reference numerals to designate the structures which are common to both FIGS. 1 and 2, includes a red active pixel sensor cell 52, a green active pixel sensor cell 54, and a blue active pixel sensor cell 56.
As further shown in FIG. 2, red cell 52 only differs from cells 12 of FIG. 1 in that red cell 52 includes a red filter 60 which only passes red photons. Similarly, green and blue cells 54 and 56 only differ from cells 12 in that green and blue cells 54 and 56 include green and blue filters 62 and 64, respectively, which only pass green and blue photons.
Thus, cells 52, 54, and 56 operate the same as cells 12 of FIG. 1 except that the information provided by red, green, and blue cells 52, 54, and 56 is limited to the intensities of the red, green, and blue lights, respectively.
One of the difficulties with the use of color pixel 50, however, is that the minority carriers formed in blue cell 56 are substantially more likely to be lost to recombination than the minority carriers formed in red and green cells 52 and 54.
This difference in recombination rates is due to the relatively shallow penetration depths of the blue photons, the higher majority carrier concentration that exists in n+ region 18 than exists in substrate 16 (or the epi layer), and the depth of the junction.
FIG. 3A shows a cross-sectional diagram that illustrates a conventional n+/p- CCD photodiode 70, while FIG. 3B shows a conventional n+/p- CMOS photodiode 80. As shown in FIGS. 3A and 3B, the typical junction depth of a CCD photodiode is approximately one micron, while the typical junction depth of a CMOS photodiode based on a 0.25 micron photolithographic process is approximately 0.1 microns. In addition, blue photons typically penetrate about 0.2 microns into the photodiode before interacting with the lattice to form electron-hole pairs (EHPs).
In the case of CCD photodiodes, the average penetration depth of the blue photons means that the majority of the EHPs are formed in the n+ region, whereas with CMOS photodiodes the majority of blue EHPs are formed in the substrate.
As noted above, the photogenerated electrons formed in the n+ region remain in the n+ region, while the photogenerated holes formed in the n+ region diffuse over to the junction where the holes are swept into the substrate under the influence of the junction electric field. However, with the high concentration of majority carriers in the n+ region, a high percentage of the holes in the CCD photodiode are lost to recombination in the n+ region before being collected by the substrate.
As also noted above, the photogenerated holes formed in the substrate remain in the substrate, while the photogenerated electrons formed in the substrate diffuse over to the junction where the electrons are swept into the n+ region under the influence of the junction electric field.
However, since the majority carrier concentration in the substrate is much lower than the majority carrier concentration in the n+ region, far fewer photogenerated electrons are lost to recombination when diffusing to the n+ region in the CMOS photodiode than photogenerated holes are lost when diffusing to the substrate in the CCD photodiode. Thus, CMOS photodiodes provide a substantially better blue response than CCD photodiodes.
However, even though CMOS photodiodes provide a substantially better blue response than CCD photodiodes, the blue response of a CMOS photodiode remains substantially below the red and green responses of a CMOS photodiode. The poor blue response of CMOS photodiodes exists because, even though the average penetration depth of a blue photon is approximately 0.2 microns, a large number of the blue photons fail to penetrate beyond the 0.1 micron junction. As a result, a large number of these photons are still lost to recombination.
On the other hand, the average penetration depth of red and green photons is on the order of several microns. At this depth, the EHPs formed from the red and green photons are almost exclusively formed in the substrate rather than the n+ region.
Thus, whereas a significant number of blue photons continue to form EHPs in the n+ region, virtually none of the red and green photons form EHPs in the n+ region. As a result, far fewer of the holes formed from red and green photons are lost to recombination in the n+ region. Thus, the blue response in a CMOS photodiode is substantially lower than the red and green responses.
One technique for equalizing the red, green, and blue responses is to increase the amplification provided by the column sense amplifiers 14 that correspond with the blue cells 56. For example, if the blue response is 10.times. lower than the red and green responses, equalized red, green, and blue responses are obtained if the column sense amplifiers 14 that correspond with the red and green cells 52 and 54 are set to provide unity gain, and the column sense amplifiers 14 that correspond with the blue cells 56 are set to provide a gain of 10.times..
One problem with increasing the amplification provided by the column sense amplifiers 14, however, is that noise is introduced by the charge-to-voltage conversion process (which varies from cell to cell) as well as by the sense amplifiers themselves. As a result, the noise is gained up by 10.times. along with the blue signal which, in turn, reduces the signal-to-noise ratio of the blue signal accordingly.
Thus, there is a need for a color pixel that increases the blue response with respect to the red and green responses while minimizing the amplification required by the column sense amplifiers.